In order for a transistor to provide amplification, it must be able to accept an input signal and produce an output signal that is greater than the input. The input signal controls current flow in the transistor. This, in turn, controls the voltage through the load.
The transistor circuit is designed to take voltage from an external power source (VCC) and apply it to a load resistor (RL) in the form of an output voltage. The output voltage is controlled by a small input voltage.
Configurations of Amplifier
The transistor is used primarily as an amplifying device. However, there is more than one way of achieving this amplification. The transistor can be connected in three different circuit configurations. The three circuit configurations are the common-base circuit, the common-emitter circuit, and the common-collector circuit.
In each configuration, one of the transistor’s leads serves as a common reference point and the other two leads serve as input and output connections. Each configuration can be constructed using either NPN or PNP transistors. In each, the transistor’s emitter-base junction is forward biased, while the collector-base junction is reverse biased. Each configuration offers advantages and disadvantages.
In the common-base circuit (Figure 1) the input signal enters the emitter-base circuit, and the output leaves from the collector-base circuit. The base is the element common to both the input and output circuits.
In the common-emitter circuit (Figure 2) the input signal enters the base-emitter circuit, and the output signal leaves from the collector-emitter circuit. The emitter is common to both the input and output circuit. This method of connecting a transistor is the most widely used.
The third type of connection (Figure 3) is the common-collector circuit. In this configuration, the input signal enters the base-collector circuit, and the output signal leaves from the emitter-collector circuit. Here, the collector is common to both the input and output circuits. This circuit is used as an impedance-matching circuit.
|CIRCUIT TYPE||INPUT RESISTANCE||OUTPUT RESISTANCE||VOLTAGE GAIN||CURRENT GAIN||POWER GAIN|
|COMMON BASE||Low||High||High||Less than 1||Medium|
|COMMON COLLECTOR||High||Low||Less than 1||Medium||Medium|
Figure 4 charts the input-output resistance and voltage, current, and power gains for the three circuit configurations. Figure 5 shows the phase relationship of input and output wave-forms for the three configurations. Note that the common-emitter configuration provides a phase reversal of the input-output signal.
Biasing of Amplifer
The basic configurations of transistor amplifier circuits are the common base, the common emitter, and the common collector. All require two voltages for proper biasing. The base-emitter junction must be forward biased and the base-collector junction must be reverse biased.
However, both bias voltages can be provided from a single source. Because the common-emitter circuit configuration is the most often used, it is described in detail here. The same principles apply to the common-base and common-collector circuits.
Common Emitter Transistor Amplifier
Figure 6 shows a common-emitter transistor amplifier using a single voltage source. The circuit is schematically diagrammed in Figure 7. The voltage source is identified as VCC. The ground symbol is the negative side of the voltage source VCC.
The single voltage source provides proper biasing for the base-emitter and base-collector junctions. The two resistors (RB and RL) are used to distribute the voltage for proper operation. Resistor RL, the collector load resistor, is in series with the collector.
When a collector current flows, a voltage develops across resistor RL. The voltage dropped across resistor RL and the voltage dropped across the transistor’s collector-to-emitter junction must add up to the total voltage applied.
Resistor RB, connected between the base and the voltage source, controls the amount of current flowing out of the base. The base current flowing through resistor RB creates a voltage across it. Most of the voltage from the source is dropped across it. A small amount of voltage drops across the transistor’s base-to-emitter junction, providing the proper forward bias. The single voltage source can provide the necessary forward-bias and reverse-bias voltages.
For an NPN transistor, the transistor’s base and collector must be positive with respect to the emitter. Therefore, the voltage source can be connected to the base and the collector through resistors RB and RL. This circuit is often called a base-biased circuit, because the base current is controlled by resistor RB and the voltage source.
The input signal is applied between the transistor’s base and emitter or between the input terminal and ground. The input signal either aids or opposes the forward bias across the emitter junction. This causes the collector current to vary, which then causes the voltage across RL to vary. The output signal is developed between the output terminal and ground.
The circuit shown in Figure 6 is unstable, because it cannot compensate for changes in the bias current with no signal applied. Temperature changes cause the transistor’s internal resistance to vary, which causes the bias currents to change. This shifts the transistor operating point, reducing the gain of the transistor. This process is referred to as thermal instability.
It is possible to compensate for temperature changes in a transistor amplifier circuit. If a portion of the unwanted output signal is fed back to the circuit input, the signal opposes the change. This is referred to as degenerative or negative feedback (Figure 8).
In a circuit using degenerative feedback, the base resistor RB is connected directly to the collector of the transistor. The current flowing through resistor RB is determined by the voltage at the collector.
If the temperature increases, the collector current increases, and the voltage across RL increases. The collector-to-emitter voltage decreases, reducing the voltage applied to RB. This reduces the base current, which causes the collector current to decrease. This is referred to as a collector feedback circuit.
Figure 9 shows another type of feedback. The circuit is similar to the one shown in Figure 7, except that a resistor RE is connected in series with the emitter lead. Resistors RB and RE and the transistor’s emitter-base junction are connected in series with the source voltage VCC.
An increase in temperature causes the collector current to increase. The emitter current then also increases, causing the voltage drop across resistor RE to increase and the voltage across resistor RB to decrease. The base current then decreases, which reduces both the collector current and the emitter current.
Because the feedback is generated at the transistor’s emitter, the circuit is called an emitter feedback circuit.
There is a problem with this type of circuit because an AC input signal develops across resistor RE as well as across the load resistor RL and the transistor. This reduces the overall gain of the circuit. With the addition of a capacitor across the emitter resistor RE (Figure 10), the AC signal bypasses resistor RE.
The capacitor is often referred to as a bypass capacitor. The bypass capacitor prevents any sudden voltage changes from appearing across resistor RE by offering a lower impedance to the AC signal. The bypass capacitor holds the voltage across resistor RE steady, while at the same time not interfering with the feedback action provided by resistor RE.
A voltage-divider feedback circuit(Figure 11) offers even more stability. This circuit is the one most widely used. Resistor RB is replaced with two resistors, R1 and R2.
The two resistors are connected in series across the source voltage VCC. The resistors divide the source voltage into two voltages, forming a voltage divider. Resistor R2 drops less voltage than resistor R1. The voltage at the base, with respect to ground, is equal to the voltage developed across resistor R2.
The purpose of the voltage divider is to establish a constant voltage from the base of the transistor to ground. The current flow through resistor R2 is toward the base. Therefore, the end of resistor R2 attached to the base is positive with respect to ground.
Because the emitter current flows up through resistor RE, the voltage dropped across resistor RE is more positive at the end that is attached to the emitter. The voltage developed across the emitter-base junction is the difference between the two positive voltages developed across resistor R2 and resistor RE. For the proper forward bias to occur, the base must be slightly more positive than the emitter.
When the temperature increases, the collector and emitter currents also increase. The increase in the emitter current causes an increase in the voltage drop across the emitter resistor RE. This results in the emitter becoming more positive with respect to ground. The forward bias across the emitter-base junction is then reduced, causing the base current to decrease. The reduction in the base current reduces the collector and emitter currents.
The opposite action takes place if the temperature decreases: The base current increases, causing the collector and emitter currents to increase.
The amplifier circuits discussed so far have been biased so that all of the applied AC input signal appears at the output. Except for a higher voltage, the output signal is the same as the input signal.
An amplifier that is biased so that the current flows throughout the entire cycle is operating as a class A amplifier.
An amplifier that is biased so that the output current flows for less than a full cycle but more than a half cycle is operating as a class AB amplifier. More than half but less than the full AC input signal is amplified in the class AB mode.
An amplifier that is biased so that the output current flows for only half of the input cycle is operating as a class B amplifier. Only one-half of the AC input signal is amplified in the class B mode.
An amplifier that is biased so that the output current flows for less than half of the AC input cycle is operating as a class C amplifier. Less than one alternation is amplified in the class C mode.
Class A amplifiers are the most linear of the types mentioned. They produce the least amount of distortion. They also have lower output ratings and are the least efficient. They find wide application where the full signal must be maintained, as in the amplification of audio signals in radios and televisions. However, because of the high power handling capabilities required for class A operation, transistors are usually operated in the class AB or class B mode.
Class AB, B, and C amplifiers produce a substantial amount of distortion. This is because they amplify only a portion of the input signal. To amplify the full AC input signal, two transistors are needed, connected in a push-pull configuration (Figure 28–16).
Class B amplifiers are used as output stages of stereo systems and public address amplifiers, and in many industrial controls.
Class C amplifiers are used for high-power amplifiers and transmitters where only a single frequency is amplified, such as the RF (radio frequency) carrier used for radio and television transmission.
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